4.3 Photosynthesis in Detail - PBS Biology

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4.3 Photosynthesis in Detail Key Concept Photosynthesis requires a series of chemical reactions.

The first stage of photosynthesis captures and transfers energy.

In Section 4.2 you read a summary of photosynthesis. Now, we will look at the process more closely. During the light-dependent reactions, energy is captured from sunlight and moved along the thylakoid membrane. This process involves two groups of molecules called photosystems. The two photosystems are called photosystem I* and photosystem II*. Both photosystems absorb energy from sunlight.

Photosystem II and Electron Transport

N o te :

In photosystem II, several things happen.

? Chlorophyll and other light-absorbing molecules absorb energy from sunlight. The energy is transferred to electrons (e?) that leave the chlorophyll. These high-energy electrons enter the electron transport chain, which is made of proteins in the thylakoid membrane.

? Water molecules are split apart into oxygen, hydrogen ions, and

The photosystems are named I and II because they were discovered in that order. The numbers do not refer to the order of the process. In other words, photosystem I does not happen first.

electrons. The oxygen is released--the same oxygen you breathe.

The electrons from water take the place of the electrons that left

the chlorophyll. ? Electrons in the electron transport chain move from protein to

thylakoid

protein. Their energy pumps hydrogen ions (H+) across the

thylakoid membrane, from outside the thylakoid to inside. The

transport of H+ ions across the membrane makes the inside of

the thylakoid have a higher concentration of H+ ions than on

the outside of the thylakoid membrane. When there is a differ-

ence in the concentration of a substance--like there is here with

H+ ions--it is called a concentration gradient.

Light-dependent reactions take place in and across the thyla-

Photosystem I and Energy-Carrying Molecules

koid membrane.

In photosystem I, several things happen.

? The electrons from photosystem II move on to photosystem I. In addition, chlorophyll absorbs energy from sunlight, which results in even more high-energy electrons.

? The electrons are added to NADP+, a molecule that is similar to ADP. This makes NADPH, a molecule that acts a lot like ATP.

* Academic Vocabulary I and II part of a number system called Roman numerals, I = 1 and II = 2

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? The concentration gradient in the thylokoid, described previously, provides the energy to make ATP. The H+ ions will diffuse, or flow, back out of the thylakoid through a channel* in the membrane. This channel is part of an enzyme called ATP synthase. As the H+ ions flow through ATP synthase, phosphate groups are added to ADP to make ATP. The energy from both ATP and NADPH is used later to make sugars.

Light-Dependent Reactions

Photosystem II and electron transport

Photosystem I and energy-carrying molecules

H+

1 Energy is absorbed from sunlight. inside the thylakoid thylakoid membrane stroma

3 Hydrogen ions are transported across the thylakoid membrane.

H+ H+

4 Energy is absorbed from sunlight. H+ H+

e-

e-

e-

e-

e-

e-

H+

H+

H+

H+

H+

H+

6 Hydrogen ions diffuse through a protein channel.

chlorophyll

2 Water molecules e-

are broken down.

2e- 2H+

H2O

1 2

O2

H+ H+

electron transport chain in the thylakoid membrane

+P

2e-

ADP

NADP+ NADPH

5 NADPH is produced when electrons are added to NADP+.

ATP synthase H+

ATP

7 ADP is changed into

ATP when hydrogen ions flow through ATP synthase.

Circle the part of the figure above that shows ADP turning into ATP when H+ ions flow through ATP synthase.

The second stage of photosynthesis uses energy from the first stage to make sugars.

The second stage of photosynthesis is the light-independent reactions. This stage also happens in the chloroplast--but in the stroma, not in the thylakoids. This stage is light-independent because it does not need light. These reactions use energy from ATP and NADPH to run the chemical reactions that make up the Calvin cycle. The chemical reactions of the Calvin cycle use carbon dioxide (CO2) and energy from ATP and NADPH to make sugars.

stroma (fluid)

Light-independent reactions take place in the stroma.

* Academic Vocabulary channel a passage, or a space that can be moved through

60 McDougal Littell Biology

The Calvin Cycle

1 Carbon dioxide

6 NADPH 6 NADP+ 6 ADP

(CO2) molecules enter the cycle 6 ATP and are added to

6CCC

five-carbon molecules. Six-carbon molecules

3 CCCCCC

CCC

are formed. 3 C

from CO2

3CCCCC

5CCC

2 Energy is added. The six-carbon molecules split to form three-carbon molecules. More energy is added and the molecules are rearranged into higher-energy molecules.

CCC

3

C C C C CC

A high-energy threecarbon molecule exits for every 3 CO2 molecules that enter. After 2 threecarbon molecules have exited, they bond to form 1 six-carbon sugar.

3 ADP 3 ATP

4 Three-carbon molecules are changed back to five-carbon molecules by energy from ATP.

Photosynthesis is important for many reasons. Plants produce food for themselves and for other organisms. They use the sugars from photosynthesis to build carbohydrates necessary for plant growth and development. Photosynthesis also removes carbon dioxide from Earth's atmosphere and produces the oxygen that you breathe.

Circle the product of the Calvin cycle in the diagram above.

4.3 Vocabulary Check

Mark It Up

photosystem electron transport chain

ATP synthase Calvin cycle

1. Which part of photosynthesis produces sugars?

Go back and highlight each sentence that has a vocabulary word in bold.

2. What molecule adds a phosphate group to ADP to make ATP?

3. What groups of molecules are involved in photosynthesis?

4. What is the name for the set of proteins in the thylakoid membrane that electrons move through?

4.3 The Big Picture

5. In which stage of photosynthesis does electron transport begin? In which stage is sunlight needed?

6. What is the main product of the Calvin cycle? 7. What are the reactants of photosynthesis? What are the products?

Interactive Reader 61

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